Silver Disk in the sky over San Mateo , Ca.

We live near Chrystal Springs reservoir, 20 miles south of San Francisco and 10 miles from the coast, where the northern most tracking site for Vandenberg is located. The sighting occurred when I was Twelve years old. So I'm not sure of the date. Being a former airline pilot, I'm basing my retrospective approximations on the observation skills acquired in this field. I was in the front of our house with my Aunt and two younger female cousins. Overhead we witnessed a disk shaped object. While not knowing the size, It appeared to be at a high altitude (over 5,000 ft.). It flew straight an level on a heading of 270, then performed 3 or 4 barrel rolls, shooting vertically at a high rate of speed out of sight. My cousins were getting upset and were told to go in the house. The next morning I asked my Aunt about the occurrence. She responded with. "Don't talk about that, you'll scare the girls." (referring to my cousins). My father explained to me that it was likely a weather balloon reflecting off the moon. I recently recalled the event after learning about the Vandenberg Atlas Missile test on 9/15 that same month. I also witnessed this test. The missile appeared to have a north west track. It left behind a beautiful bright purple plume that lingered for several hours. I thought it was interesting that the two events happened within such close proximity of each other.